docs: schemas: Extract disk source host specification
docs: schemas: Extract disk source host specification

'diskSourceNetwork' schema define was rather big and it would be hard to
simplify it. Split out the host portion subelement into a separate
define.

fabianfreyer/libvirt,andreabolognani/libvirt,eskultety/libvirt,fabianfreyer/libvirt,jardasgit/libvirt,datto/libvirt,nertpinx/libvirt,andreabolognani/libvirt,jfehlig/libvirt,olafhering/libvirt,jardasgit/libvirt,fabianfreyer/libvirt,zippy2/libvirt,jfehlig/libvirt,jardasgit/libvirt,fabianfreyer/libvirt,jardasgit/libvirt,andreabolognani/libvirt,zippy2/libvirt,olafhering/libvirt,eskultety/libvirt,olafhering/libvirt,eskultety/libvirt,datto/libvirt,crobinso/libvirt,VenkatDatta/libvirt,jfehlig/libvirt,olafhering/libvirt,libvirt/libvirt,jfehlig/libvirt,VenkatDatta/libvirt,datto/libvirt,VenkatDatta/libvirt,andreabolognani/libvirt,nertpinx/libvirt,andreabolognani/libvirt,libvirt/libvirt,crobinso/libvirt,VenkatDatta/libvirt,jardasgit/libvirt,zippy2/libvirt,eskultety/libvirt,fabianfreyer/libvirt,libvirt/libvirt,crobinso/libvirt,nertpinx/libvirt,datto/libvirt,VenkatDatta/libvirt,nertpinx/libvirt,nertpinx/libvirt,zippy2/libvirt,datto/libvirt,eskultety/libvirt,crobinso/libvirt,libvirt/libvirt